-LRB- CNN -RRB- -- Coast Guard aircraft have found no sign of a spill from a Royal Dutch Shell oil drilling barge that ran aground off a southern Alaska island during a fierce winter storm , authorities reported Tuesday .

The 266-foot Kulluk `` is sound . There is no sign of a breach of the hull . There is no sign of a release of any product , '' Coast Guard Capt. Paul Mehler III told reporters Tuesday afternoon .

Mehler said the Kulluk had about 143,000 gallons of diesel fuel and 12,000 gallons of combined lube oil and hydraulic fluid on board . The rig -- a key part in Shell 's controversial Arctic oil exploration project -- ran aground off uninhabited Sitkalidak Island , about 200 miles south of Anchorage , on Monday night .

The Kulluk had been working in the Beaufort Sea , off Alaska 's North Slope , until October . It was being towed back to its winter home in Seattle when it ran into a severe storm off the Alaskan coast . The Coast Guard evacuated its 18-man crew Saturday night , and it drifted for 10 hours on Sunday after the tug that was towing it lost power .

Monday night , tug crews had to cut the rig loose during a storm that whipped up 24-foot waves , leading to its grounding .

Crews battle harsh weather trying to tow drilling unit to Alaska harbor

Sean Churchfield , Shell 's Alaska operations manager , said Tuesday that the rig `` is upright , rocking with a slow motion , and is stable '' -- but he added , `` There is still a lot of work to be done to bring this to a safe conclusion . ''

Three people suffered minor injuries over the past few days , but have already returned to work , Churchfield said .

The rig was aground in an area of Ocean Bay , where water depth is 32 feet to 48 feet . A joint command has been set up to handle salvage efforts , but ongoing bad weather has kept crews from getting aboard the vessel , Mehler said .

Weather conditions were expected to improve through the rest of the week , with seas subsiding from 24 feet Tuesday to 11 feet by Friday , according to the National Weather Service .

Most of the nearby shore is owned by a native Alaskan corporation on adjacent Kodiak Island , said Steven Russell , of Alaska 's Department of Environmental Conservation . State officials are working with residents to watch out for any environmental damage from the grounding , he said . As of Tuesday , `` we have no indications of environmental or wildlife impact , '' Russell said .

Shell 's Arctic exploration plans caused widespread concern among environmentalists and were held up after BP 's Deepwater Horizon disaster in the Gulf of Mexico . Eskimo activist Caroline Cannon , one of Shell 's toughest critics , said she fears a spill from the Kulluk could harm fish and wildlife in the area .

Shell rig accident off Alaska shows dangers of drilling in Arctic

`` It 's a scary thought you know , because our food chain is out there . Our people rely on our food , '' she said .

Shell says it 's working at far less depth and lower pressures than the BP well that erupted off Louisiana , killing 11 men aboard and unleashing an undersea gusher that took three months to cap . The fuel on board the rig is used to power equipment and is not the result of the drilling operations Shell conducted off the North Slope , on the opposite side of the vast state from where the Kulluk now rests .

The U.S. Geological Survey estimates more than 90 billion barrels of oil and nearly 1,700 trillion cubic feet of natural gas may be recoverable by drilling . And the shrinking of the region 's sea ice -- which hit record lows in 2012 -- has created new opportunities for energy exploration in the region .

Climate researchers say that a decrease in sea ice is a symptom of a warming climate , caused largely by the combustion of carbon-rich fossil fuels . The science is politically controversial but generally accepted as fact by most scientists .

CNN 's Paul Vercammen contributed to this report .

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NEW : Alaska official says the state is looking for signs of environmental damage

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The grounded drilling barge Kulluk shows no sign of leaking fuel , the Coast Guard says

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A tug had to set the rig free during a severe storm Monday

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The rig 's crew of 18 evacuated on Saturday